It is in possession of a female Enclave scientist, located in the southeast research hangar of the base. This is missable as when you press on to the Mobile Base Crawler, the constant spawning of corpses from the battlefield will result in in the scientist disappearing.
Little Sister, I've stashed away a considerable amount of fuel behind the small building west of the Air Traffic Control Tower as you requested.
The container's tucked away under some barrels and tires. And no, I couldn't safely confiscate an incinerator for you, that's your job.
I took too big a risk hiding that fuel for you in the first place. If you do come across an incinerator, please be careful.
Anyway, they're shipping me out to some relay station far west of here in the Wasteland. I'm afraid I won't be seeing you for a while.
Take care and good luck in all your endeavors. Your Big Brother.